Extreme ultraviolet lithography process and mask

ABSTRACT

A system of an extreme ultraviolet lithography (EUVL) is disclosed. The system includes a mask having first and second reflective regions. The system also includes an illumination to expose the mask to produce a resultant reflected light form the mask. The resultant reflected light is constructed by a first reflected light reflected from the first reflective region and a second reflected light reflected from the second reflective region. The resultant reflected light contains mainly diffracted light. The system also includes a projection optics box (POB) to collect and direct resultant reflected light to expose a target.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth in the past several decades. Technological advances insemiconductor materials and design have produced increasingly smallerand more complex circuits. These material and design advances have beenmade possible as the technologies related to processing andmanufacturing have also undergone technical advances. As a size of thesmallest component has decreased, numerous challenges have risen. Forexample, the need to perform higher resolution lithography processesgrows. One lithography technique is extreme ultraviolet (EUV)lithography. Other techniques include X-Ray lithography, ion beamprojection lithography, electron beam projection lithography, andmultiple electron beam maskless lithography.

EUV lithography is a promising patterning technology for very smallsemiconductor technology nodes, such as 14-nm, and beyond. EUVlithography is similar to optical lithography in that it needs a mask toprint wafers, except that it employs light in the EUV region, e.g., atabout 13.5 nm. At the wavelength of 13.5 nm, most materials are highlyabsorbing. Thus, reflective optics, rather than refractive optics, arecommonly used in EUV lithography. Although existing methods of EUVlithography have been generally adequate for their intended purposes,they have not been entirely satisfactory in all respects. For example,challenges rise to obtain high optical contrast. It is desired to haveimprovements in this area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram of a lithography system for implementing oneor more embodiments of the present disclosure.

FIG. 2 is a diagrammatic cross-sectional view of various aspects of oneembodiment of a mask substrate at various stages of a lithographyprocess constructed according to aspects of the present disclosure.

FIGS. 3A-3D are diagrammatic cross-sectional views of various aspects ofone embodiment of an EUV mask at various stages of a lithography processconstructed according to aspects of the present disclosure.

FIGS. 4A and 4B are diagrammatical views of various exposing intensityprofiles during a lithography exposure process according to one or moreembodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper”, “over” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

Referring to FIG. 1, an EUV lithography system 10 that may benefit fromone or more embodiments of the present invention is disclosed. The EUVlithography process 10 employs an EUV radiation source 20 having awavelength of about 1-100 nm, including an EUV wavelength of about 13.5nm.

The EUV lithography system 10 also employs an illuminator 30. Theilluminator 30 may comprise refractive optics, such as a single lens ora lens system having multiple lenses (zone plates) and/or reflectiveoptics, such as a single mirror or a mirror system having multiplemirrors, in order to direct light from the radiation source 20 onto amask 40. In the EUV wavelength range, reflective optics is generallyemployed. Refractive optics, however, can also be realized byzoneplates. In the present embodiment, the illuminator 30 is set up toprovide an on-axis illumination to illuminate a mask 40. In on-axisillumination, most all incoming light rays incident on the mask are atthe same angle of incidence (AOI), e.g., AOI=6°, as that of a chief ray.In many situations, there may be some angular spread of the incidentlight. For example, the EUV lithography system 10 may utilize diskillumination (i.e., illumination on a pupil plane is shaped like a diskcentered at the pupil center). Partial coherence a can also be used todescribe a point source which produces a plane wave for illuminating themask 40. In the present embodiment, it is sufficient to employ a nearlyon-axis illumination having point sources with a less than 0.3.

The EUV lithography system 10 also employs a mask 40 (the terms mask,photomask, and reticle are used herein to refer to the same item). Inthe present embodiment, the mask 40 is a reflective mask. The mask 40may incorporate other resolution enhancement techniques such asphase-shifting mask (PSM) and/or optical proximity correction (OPC).

In general, an incident light ray reflected from a mask diffracts intovarious diffraction orders due to presence of these mask patterns, suchas a 0-th diffraction order ray, a −1-st diffraction order ray and a+1-st diffraction order ray. In the present embodiment, the 0-thdiffracted light rays are mostly eliminated due a structure of the mask40, which will be described in details later. The −1-st and +1-stdiffraction order are collected and directed to expose a target. Sincethe strength of the −1-st and +1-st diffraction order rays are wellbalanced, they interfere with each other and will generate a highcontrast aerial image. As an example, equipped with the mask 40, a highcontrast and uniform light intensity across a substrate is achieved, asshown in FIGS. 4A and 4B. FIGS. 4A and 4B graphically illustrate theexposure intensity distribution (vertical scale) across a photoresistlayer on a substrate to be exposed (horizontal scale). In FIG. 4B, theunit for the exposing intensity is a relative unit ranging from 0 to 1.In this case, “1” stands for 100% of the exposing intensity from theexposing system before reaching the photoresist layers.

The EUV lithography system 10 also employs optics 50. The optics 50 mayhave refractive optics or reflective optics. The radiation reflectedfrom the mask 40 (e.g., a patterned radiation) is collected by theoptics 50.

The target 60 includes a semiconductor wafer with a photosensitive layer(e.g., photoresist or resist), which is sensitive to the EUV radiation.The target 60 may be held by a target substrate stage. The targetsubstrate stage provides control of the target substrate position suchthat the image of the mask is scanned onto the target substrate in arepetitive fashion (though other lithography methods are possible).

The following description refers to the mask 40 and a mask fabricationprocess. The mask fabrication process usually includes two steps: a masksubstrate fabrication process and a mask patterning process. During themask substrate fabrication process, a mask substrate is formed bydeposing suitable layers (e.g., multiple reflective layers) on asuitable substrate. The mask substrate is patterned during the maskpatterning process to have a design of a layer of an integrated circuit(IC) device (or chip). The patterned mask is then used to transfercircuit patterns (e.g., the design of a layer of an IC device) onto asemiconductor wafer. The patterns can be transferred over and over ontomultiple wafers through various lithography processes. Several masks(for example, a set of 15 to 30 masks) may be used to construct acomplete IC device.

Referring to FIG. 2, a EUV mask substrate 100 comprises a material layer110 made of low thermal expansion material (LTEM). The LTEM includesTiO₂, doped SiO₂, and/or other low thermal expansion materials known inthe art. The LTEM layer 110 serves to minimize image distortion due tomask heating. In the present embodiment, the LTEM layer includesmaterials with a low defect level and a smooth surface. In addition, aconductive layer 105 may be deposed under (as shown in the figure) theLTEM layer 110 for the electrostatic chucking purpose. In an embodiment,the conductive layer 105 includes chromium nitride (CrN), though othercompositions are possible.

The EUV mask substrate 100 includes a reflective multilayer (ML) 120deposed over the LTEM material layer 110. According to Fresnelequations, light reflection will occur when light propagates across theinterface between two materials of different refractive indices. Thereflected light is larger when the difference of refractive indices islarger. To increase the reflected light, one may also increase thenumber of interfaces by deposing a multilayer of alternating materialsand let light reflected from different interfaces interfereconstructively by choosing appropriate thickness for each layer insidethe multilayer. However, the absorption of the employed materials forthe multilayer limits the highest reflectivity that can be achieved. Thereflective ML 120 includes a plurality of film pairs, such asmolybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum aboveor below a layer of silicon in each film pair). Alternatively, thereflective ML 120 may include molybdenum-beryllium (Mo/Be) film pairs,or any material that is highly reflective at EUV wavelengths can beutilized for the reflective ML 120. The thickness of each layer of thereflective ML 120 depends on the EUV wavelength and the incident angle.The thickness of the reflective ML 120 is adjusted to achieve a maximumconstructive interference of the EUV light reflected at each interfaceand a minimum absorption of the EUV light by the reflective ML 120. Thereflective ML 120 may be selected such that it provides a highreflectivity to a selected radiation type/wavelength. A typical numberof film pairs is 20-80, however any number of film pairs is possible.The reflective ML 120 usually achieves a reflectance of 0.65 or above.In an embodiment, the reflective ML 120 includes forty pairs of layersof Mo/Si. Each Mo/Si film pair has a thickness of about 7 nm, with atotal thickness of 280 nm. In this case, a reflectivity of about 70% isachieved.

The EUV mask substrate 100 may also include a capping layer 130 disposedabove the reflective ML 120 to prevent oxidation of the reflective ML.In one embodiment, the capping layer 130 includes ruthenium (Ru), Rucompounds such as RuB, RuSi, chromium (Cr), Cr oxide, and Cr nitride.

One or more of the layers 105, 120 and 130 may be formed by variousmethods, including physical vapor deposition (PVD) process such asevaporation and DC magnetron sputtering, a plating process such aselectrode-less plating or electroplating, a chemical vapor deposition(CVD) process such as atmospheric pressure CVD (APCVD), low pressure CVD(LPCVD), plasma enhanced CVD (PECVD), or high density plasma CVD (HDPCVD), ion beam deposition, spin-on coating, metal-organic decomposition(MOD), and/or other methods known in the art.

Referring to FIGS. 3A-3D, a first reflective region 210 and a secondreflective region 220 are formed over the EUV mask substrate 100 for anEUV mask 200. In the present embodiment, the first reflective region 210and the second reflective region 220 are configured such that when anincoming light 300 is reflected from the first reflective region 210,referred as to a first reflected light 310 has an about 180 degree phaseshift and a substantially equal intensity with respect to a reflectedlight from the second reflective region 220, referred as to a secondreflected light 320 The configuration may be referred to as a 2^(nd)order Kinoform grating. Thus, a resultant reflected light constructed bythe first reflected light 310 and second reflected light 320 containsalmost zero 0-th diffraction order. The resultant reflected lightcontain mainly −1-st and +1-st diffraction.

In one embodiment, the first reflective region 210 includes a molybdenum(Mo) layer 212 deposited over the capping layer 130, having a thicknessof about 42.53 nm. While the second reflective region 220 includes analuminum (Al) layer 222 deposited over the capping layer 130, having athickness of about 16 nm, as shown in FIG. 3A. Alternatively, instead ofan Al layer, an actinium (Ac) layer 222 is deposited over the cappinglayer 130, having a thickness of about 6 nm.

In another embodiment, the first reflective region 210 includes a Molayer 214 deposited over the capping layer 130, having a thickness ofabout 14.53 nm. While the second reflective region 220 is formed by theML trench 224 of, having a depth d of 28 nm. The trench 224 is formed byremoving the capping layer 130 and a portion of the ML 120 in the secondreflected region 220, as shown in FIG. 3B.

In yet another embodiment, the first reflective region 210 is formed bythe capping layer 130 disposed over the ML 120 and the ML 120 over theLTEM layer 110. While the second reflective region 220 is formed by anAL layer 222 disposed over the ML trench 224. The Al layer 222 has athickness of about 16 nm as shown in FIG. 3C. Alternatively, instead ofAl layer, an Ac layer 222 is deposited over the ML trench 224, having athickness of about 6 nm.

In yet another embodiment, the first reflective region 210 includes theAl layer 222 deposited over the Mo layer 212, having a thickness ofabout 16 nm, and the Mo layer 212 deposited over the capping layer 130,having a thickness of about 32.53 nm. While the second state 220includes the Al layer 222 deposited over the ML trench 224, as shown inFIG. 3D. In one embodiment, the Al layer 222 is formed simultaneouslyover the first and second reflective regions, 210 and 220, by localdeposition process, such as a gas-assisted focused-electron-beam-induceddeposition. Alternatively, instead of Al layer, an actinium (Ac) layer222 is deposited over the Mo layer 212 in the first reflective region210 and over the ML trench 224 in the second reflective region 220,having a thickness of about 6 nm.

Based on the above, the present disclosure offers the EUV lithographysystem and process employing an EUV mask having a structure to eliminate0-th diffracted light ray in a resultant reflected light reflected fromthe mask. The resultant reflected light contains mainly of −1-st and+1-st diffraction order light. The EUV lithography system and processdemonstrates an enhancement of aerial image contrast, image fidelity andquality. As an example, by using the system and the process provided inthe present embodiment, an image with a good contrast and quality isachieved, as shown in FIGS. 4A and 4B.

The present disclosure is directed towards lithography systems andprocesses. In one embodiment, an extreme ultraviolet lithography (EUVL)system includes a mask having first and second reflective regions. Thesystem also includes an illuminator to expose the mask to produce aresultant reflected light form the mask. The resultant reflected lightis constructed by a first reflected light reflected from the firstreflective region and a second reflected light reflected from the secondreflective region. The resultant reflected light contains mainlydiffracted light. The system also optics to collect and direct resultantreflected light to expose a target.

In another embodiment, an EUVL process includes receiving a mask. Themask has first and second reflective regions. The process also includesexposing the mask by an illumination to produce a resultant reflectedlight from the first and the second reflective regions. The resultantreflected light contains almost zero non-diffracted light. The systemalso includes collecting and directing reflected light from theresultant reflective light by a projection optics box (POB) to expose atarget.

The present disclosure is also directed towards masks. In oneembodiment, the mask for extreme ultraviolet lithography (EUVL) includesa low thermal expansion material (LTEM) material layer, a reflectivemultilayer (ML) above one surface of the LTEM material layer. The maskalso includes a first and a second reflective regions formed over thereflective ML. The first and second reflective regions are configuredsuch that a reflected light from the first reflective region has anabout 180 degree phase shift respect to the reflected light from thesecond reflective region.

The foregoing outlined features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An extreme ultraviolet (EUV) lithography system,comprising: a mask having first and second reflective regions, whereinthe mask includes: a reflective multilayer; a first metal-containinglayer disposed on the reflective multilayer in the first reflectiveregion, wherein the second reflective region is free of the firstmetal-containing layer; and a second metal-containing layer disposed onthe reflective multilayer in the second reflective region, wherein thesecond metal-containing layer is different in composition from the firstmetal-containing layer, and wherein the second metal-containing layerdoes not extend between the first metal-containing layer and thereflective multilayer in the first reflective region; an illuminator toexpose the mask to produce a resultant reflected light including a firstreflected light reflected from the first reflective region and a secondreflected light reflected from the second reflective region, wherein theresultant reflected light includes diffracted light; and optics tocollect and direct the resultant reflected light towards a target. 2.The system of claim 1, wherein the mask includes: a low thermalexpansion material (LTEM)_layer in both the first and second reflectiveregions; a conductive layer disposed on a first surface of the LTEMlayer in both the first and second reflective regions, wherein thereflective multilayer (ML) is disposed on a second surface of the LTEMlayer in both the first and second reflective regions.
 3. The system ofclaim 1, further comprising: a capping layer disposed above thereflective multilayer in both the first and second reflective regions.4. The system of claim 1, wherein the first metal-containing layerincludes molybdenum (Mo); and wherein the second metal-containing layerincludes aluminum (Al).
 5. The system of claim 1, wherein the firstmetal-containing layer includes molybdenum (Mo); and wherein the secondmetal-containing layer includes actinium (Ac).
 6. The system of claim 1,further comprising: a capping layer disposed on the reflectivemultilayer in the first reflective region, wherein the second reflectiveregion is free of the capping layer.
 7. The system of claim 1, whereinthe first metal-containing layer includes molybdenum (Mo); and wherein afirst portion of the reflective multilayer in the first reflectiveregion has a first thickness and a second portion of the reflectivemultilayer in the second reflective region has a second thickness thatis less than the first thickness.
 8. The system of claim 1, wherein afirst portion of the reflective multilayer in the first reflectiveregion has a first thickness and a second portion of the reflectivemultilayer in the second reflective region has a second thickness thatis less than the first thickness; and wherein the secondmetal-containing layer includes aluminum (Al).
 9. The system of claim 1,wherein a first portion of the reflective multilayer in the firstreflective region has a first thickness and a second portion of thereflective multilayer in the second reflective region has a secondthickness that is less than the first thickness; and wherein the secondmetal-containing layer includes actinium (Ac).
 10. The system of claim1, wherein a first portion of the reflective multilayer in the firstreflective region has a first thickness and a second portion of thereflective multilayer in the second reflective region has a secondthickness that is less than the first thickness; wherein the firstmetal-containing layer includes molybdenum (Mo) and the firstmetal-containing layer has a thickness of about 32.53 nm; and whereinthe second metal-containing layer includes aluminum (Al), the secondmetal-containing layer has a thickness of about 16 nm, and the secondmetal-containing layer is further disposed over the firstmetal-containing layer in the first reflective region.
 11. The system ofclaim 1, wherein a first portion of the reflective multilayer in thefirst reflective region has a first thickness and a second portion ofthe reflective multilayer in the second reflective region has a secondthickness that is less than the first thickness; wherein the firstmetal-containing layer includes molybdenum (Mo) and the firstmetal-containing layer has a thickness of about 32.53 nm; and whereinthe second metal-containing layer includes actinium (Ac), the secondmetal-containing layer has a thickness of about 6 nm, and the secondmetal-containing layer is further disposed over the firstmetal-containing layer in the first reflective region.
 12. The system ofclaim 1, wherein the resultant reflected light contains almost zeronon-diffracted light.
 13. The system of claim 1, wherein the firstmetal-containing layer is disposed at a different height than the secondmetal-containing layer.
 14. The system of claim 1, wherein a sidewall ofthe first metal-containing layer interfaces with a sidewall of thesecond metal-containing layer.
 15. A mask for extreme ultravioletlithography (EUVL), comprising: a low thermal expansion material (LTEM)layer; a reflective multilayer (ML) above one surface of the LTEM layer;a first and a second reflective regions formed over the reflective ML,wherein a reflected light from the first reflective region has an about180 degree phase shift with respect to a reflected light from the secondreflective region; a first metal-containing layer disposed on thereflective ML in the first reflective region, wherein the firstmetal-containing layer does not extend into the second reflectiveregion; and a second metal-containing layer disposed on the reflectiveML in the second reflective region, wherein the second metal-containinglayer does not extend between the first metal-containing layer and thereflective ML in the first reflective region.
 16. The mask of claim 15,further comprising: a capping layer above the reflective ML in both thefirst and second reflective regions; wherein the first metal-containinglayer includes molybdenum (Mo); and wherein the second metal-containinglayer includes actinium (Ac).
 17. The mask of claim 15, wherein thesecond metal-containing layer includes aluminum (Al).
 18. The mask ofclaim 15, further comprising: a capping layer above the reflective ML inthe first reflective region; wherein the first metal-containing layerincludes molybdenum (Mo); and wherein a first portion of the reflectiveML in the first reflective region has a first thickness and a secondportion of the reflective ML in the second reflective region has asecond thickness that is less than the first thickness.
 19. The mask ofclaim 15, further comprising: a capping layer above the reflective ML inthe first reflective region; wherein a first portion of the reflectiveML in the first reflective region has a first thickness and a secondportion of the reflective ML in the second reflective region has asecond thickness that is less than the first thickness; and wherein thesecond metal-containing layer includes aluminum (Al).
 20. The mask ofclaim 15, further comprising: a capping layer above the reflective ML inthe first reflective region; wherein a first portion of the reflectiveML in the first reflective region has a first thickness and a secondportion of the reflective ML in the second reflective region has asecond thickness that is less than the first thickness; wherein thefirst metal-containing layer includes molybdenum (Mo); and wherein thesecond metal-containing layer includes actinium (Ac).
 21. An extremeultraviolet (EUV) lithography process, comprising: receiving a maskhaving first and second reflective regions, wherein the mask includes: areflective multilayer; a first metal-containing layer disposed on thereflective multilayer in a first region, wherein a second region is freeof the first metal-containing layer; and a second metal-containing layerdisposed on the reflective multilayer in the second region, wherein thesecond metal-containing layer is different in composition from the firstmetal-containing layer, and wherein the second metal-containing layerdoes not extend between the first metal-containing layer and thereflective multilayer in the first region; exposing the mask by anillumination to produce a resultant reflected light from the first andthe second reflective regions, wherein the resultant reflected lightincludes more diffracted light than non-diffracted light; and directingthe resultant reflective light to expose a target.
 22. The process ofclaim 21, wherein −1^(st) and +1^(st) diffraction order light, amajority of the diffracted light, is collected and directed to exposethe target.